Fuel economy of vehicles has been improved by lowering the rolling resistance of tires to suppress heat build-up. In recent years there has been a growing demand for vehicles with higher fuel economy, and thus further studies are essential to enhance fuel economy. Natural rubber, which is commonly used mainly in inner components of tires, has been considered to have higher fuel economy than styrene-butadiene rubber which is often used in treads and the like. Since the fuel economy of styrene-butadiene rubber has already been greatly improved, further reduction in fuel consumption is increasingly difficult. To achieve even higher fuel economy as the whole tire, the fuel economy of natural rubber also needs to be improved.
An example of a method for improving fuel economy by modifying natural rubber includes washing a mixture of natural rubber latex and a surfactant, as disclosed in Patent Literature 1. In this method, however, proteins and gel fraction are removed to a certain extent but to insufficient levels, and thus further reduction in tan 5 is desired. Moreover, rubber for tires is required to have such properties as heat-aging resistance. The method of Patent Literature 1 unfortunately provides insufficient heat resistance and thus needs improvement to simultaneously provide fuel economy and heat-aging resistance.
Meanwhile, natural rubber has a higher Mooney viscosity and thus lower processability than synthetic rubbers. Usually, natural rubber is mixed with a peptizing agent and then masticated to reduce the Mooney viscosity before use, which lowers productivity. Further, since the mastication breaks the molecular chains of natural rubber, the resultant natural rubber unfortunately loses high molecular weight polymer characteristics that natural rubber originally has, such as good abrasion resistance, fuel economy, rubber strength, and heat-aging resistance.